NEW YORK — Imagine a mash-up of an intense family drama along the lines of 1980’s “Ordinary People” and a foray into the occult like “Rosemary’s Baby” from 1968 and you’ll have a sense the unusual tone of “Hereditary” (A24).

What you won’t have a grasp on, however, will be exactly what’s driving the plot of this subtly creepy horror tale.

Writer-director Ari Aster’s feature debut keeps the audience guessing as a string of macabre occurrences beset an ostensibly normal family in the wake of its problematic matriarch’s death. Was Granny a witch? A worshipper of the Devil? We’re only given hints about her strange rituals.

Charlie follows in her gran’s footsteps by cutting off the head of a dead bird and making angry sketches of the people around her as if to curse them. Peter thinks he might be seeing dead people, and Annie befriends Joan (Ann Dowd), a participant in her grief recovery group who turns out to be a medium. Suggestive details and plot developments all pile up, but answers are long delayed.

Things only get worse for the Grahams after a second loss, this one much more shocking than Grandma’s demise, threatens to divide them against each other in bitterness and blaming.

Aster mostly pays patient viewers off with a resounding conclusion, and Collette is furiously intense throughout. But the gloomily atmospheric film, which initially goes easy on the gore, gets less restrained in that respect as it goes along.

This is not a good pick for those averse to hideous sights nor for those with short attention spans. Hardy horror fans, by contrast, may appreciate this slowly unfolding study in spiritual as well as emotional dysfunction.

The film contains black magic and Satanist themes, gruesome events and images, drug use, glimpses of full male and female nudity in a nonsexual context, a few profanities and numerous rough and crude terms. The Catholic News Service classification is L — limited adult audience, films whose problematic content many adults would find troubling. The Motion Picture Association of America rating is R — restricted. Under 17 requires accompanying parent or adult guardian.